In the following, prior art mixing apparatus of the pulp and paper industry have been discussed as examples of known techniques of mixing a flowing medium to another. However, it should be understood that in spite of the fact that only mixers of the pulp and paper industry have been discussed, it has not been done for the purpose of limiting the scope of the present invention to these fields of industry.
A widely used example of chemical mixers for pulp has been discussed in U.S. Pat. No. 5,279,709, which discloses a method of treating a fiber suspension having a consistency of 5-25% in an apparatus within a fiber suspension transfer line. The apparatus comprises a chamber having an axis in the direction of flow of the fiber suspension, a suspension inlet and a suspension outlet having an axis in alignment with the chamber axis, and a fluidizing rotor having an axis of rotation transverse to the direction of flow and being disposed within the chamber for rotation therein. The rotor comprises blades, each blade having a proximal and distal end and the blades diverging from the proximal end and extending in spaced relation from the axis of rotation along an axial length thereof. The method comprises feeding the suspension from the suspension transfer line through the inlet into the chamber, introducing chemicals into the fiber suspension upstream of the fluidizing rotor, rotating the fluidizing rotor within the chamber so as to form an open center bounded by a surface of revolution and subjecting the suspension moving toward the outlet to a shear force field sufficient to fluidize the suspension, to mix the chemicals evenly into the suspension and to render the suspension flowable, flowing the suspension through the open center of the rotor, and discharging the suspension from the chamber through the suspension outlet.
The above-described mixer has found a number of imitations, of which, for example, U.S. Pat. No. 5,575,559 and U.S. Pat. No. 5,918,978 can be mentioned.
All the above-discussed mixers have a few features in common. The rotor is brought into the mixing chamber in a direction perpendicular to the axis of the flow through the mixing chamber. The rotor is formed of finger-like blades, which leave the center of the rotor open. The rotor shaft and the rotor blades are arranged such that the mixing chamber with the rotor installed does not form a symmetrical mixing space, but an asymmetrical one, where the turbulence created by the rotor is not optimal. The result is that the mixing of the chemical with the fiber suspension is not even, but in some areas of the mixer the turbulence level is higher, resulting in more even mixing than in areas where the turbulence level is lower.
There is yet another mixer where the transverse rotor construction has been used. The mixer has been discussed in EP-B2-0 606 250. Here the mixer for admixing a treatment agent to a pulp suspension having a consistency of 10-25% comprises a cylindrical housing with a mixing chamber defined between an inner wall of the cylindrical housing and a casing of a coaxially mounted, substantially cylindrical rotor provided with mixing members on its casing surface, an inlet in the housing for supplying pulp to the mixing chamber, an inlet in the housing for supplying treatment agent to the mixing chamber and an outlet for withdrawing mixed pulp and treatment agent, a mixing zone in the housing provided with stationary mixing members wherein a gap is defined between the mixing members of the rotor and the stationary mixing members. The mixing chamber and the mixing zone have a width corresponding to the axial length of the rotor. The stationary mixing members are arranged on a portion within an angle of 15-180 degrees of the inner wall of the housing. The pulp inlet and the treatment agent inlet extend along the entire width of the mixing chamber for adding the pulp and the treatment agent each in well-formed thin layers. The inlet for the treatment agent is connected to the mixing chamber at a circumferential position prior to the mixing zone. The outlet extends along the entire width of the mixing chamber, and a cylindrical surface is formed directly after the outlet to prevent pulp from flowing backward past the rotor. In other words, the mixer of the EP patent has a closed cylindrical rotor with solid mixing members on the rotor surface. The cylindrical rotor is positioned in a cylindrical mixing chamber. The basic idea in the EP document is to feed both pulp and the chemical as thin layers in the mixing zone between the rotor and the chamber wall and mix such there.
However, based on practical experiences it has been learned that the mixing is not very efficient in the narrow slot between the rotor and the mixing chamber. Also, it has been learned that the energy consumption of this type of mixer is high compared, for instance, to the mixer discussed in U.S. Pat. No. 5,279,709 mentioned first.